Recent control systems for battery powered d.c. traction motors have frequently employed silicon controlled rectifiers (SCRs) as "current chopper" control devices to provide a periodic on-off control to vary the current to the drive motor, and the resultant speed, by variation of the total average duration of "on" intervals. In this manner, the so-called duty cycle is adjusted. The SCR controls have been very satisfactory in many ways. However, they do have various problems including substantial cost, substantial energy losses, and substantial size and weight.
A promising recent entry in the solid state switching field is the power field effect transistor such as the MOSFET. The MOSFETs are metal-oxide-semiconductor field effect transistors which enjoy the advantages of very high (nearly infinite) input impedence, very fast switching times, a positive temperature coefficient of resistance, and especially low cost. The positive temperature coefficient of resistance is very advantageous for paralleling of multiple MOSFETs because it provides a degree of automatic load sharing.
MOSFETs are designed for different voltage ratings. The higher the voltage rating, the higher the internal resistance of the MOSFET and the lower the current which can be safely handled by the MOSFET at a safe rate of power dissipation. Accordingly, it is desirable to try to employ MOSFETs which have the lowest possible voltage rating, for both economy and for avoidance of unnecessary power dissipation.
Unfortunately, the MOSFETs must not only handle the operating voltage, but also any transient voltage spikes which arise from switching of the MOSFETs. These transient voltages arise especially when the MOSFETs are switched off. This means that the voltage rating of the MOSFETs must be somewhat higher than the maximum voltage of the power source. It also means that measures are preferably taken to limit the transient voltage spikes during switching operations.
Another disadvantage of prior controllers has been that the commutation frequency has typically been in the audible range, sometimes at 2,000 hertz, for instance. This leads to a substantial audible tone during operation of the system.
It has been found to be much more satisfactory to set the commutation frequency at a higher level such as 15,000 hertz, which is above the upper frequency threshold of hearing for many humans. Also, when the frequency is raised to that level, the amplitude of the sound is substantially attenuated so that even for those who can actually hear it, the sound is not a serious problem. However, when the operating frequency is raised as high as 15,000 hertz, the commutating speed for the MOSFETs must generally be higher, leading to higher transient voltage spikes.
Another problem is that it is often desired to operate vehicles which are to be controlled with a battery supply voltage which is at a nominal 36 volt level, rather than a level lower than 36 volts. This also raises the voltage which must be dealt with by the MOSFETs. Furthermore, with a vehicle of some size, such as a baggage carrying vehicle, or a golf cart, the motor current may be at a substantial level, as high as 400 amperes.
Accordingly, it is an object of the present invention to provide an improved solid state d.c. motor control which safely employs power MOSFETs having a relatively low voltage rating as the switching elements and which is extremely durable and inexpensive and compact and which is capable of operation at frequencies in the order of 15,000 hertz.
It is another object of the invention to provide an improved solid state d.c. motor control which incorporates fail-safe features which prevent undue strain on the drive motor and the battery and the control system itself.
One problem in the design of variable speed d.c. motor controls is that operators of battery powered vehicles often employ a procedure of reversing the motor connections in order to stop the vehicle rapidly. This is referred to as "plug" braking.
When plug braking is employed, the motor field is typically reversed in its connections and the strength of the field must be substantially reduced immediately in order to avoid excessive braking, leading to skidding, undue strain on the equipment, and lack of control. In order to substantially reduce the field current, the operation of the commutating MOSFETs must be reduced to a very low duty cycle. Unfortunately, it is virtually impossible to switch the MOSFETs off rapidly enough to reduce the duty cycle sufficiently for reasonable plug braking at an operating frequency of 15,000 hertz without causing excessive transient voltage spikes.
Accordingly, it is another object of the present invention to solve the above-mentioned problem by providing circuitry which detects when plug braking is called for by the operator and for changing the mode of operation of the control system to achieve the desired plug braking field current.
Further objects and advantages of the invention will be apparent from the following description and the accompanying drawings.